Characteristics Of Forest Soil Macropore And Rock Fragment And Their Effects On Soil Water In Miyun Reservoir Watershed

Soil macropore plays a key role in the transportation of water,nutrients,chemical substances and pollutants.The distribution of rock fragment has a certain effect on soil water movement.It is necessary to explore the characteristics of soil macropore and rock fragments in different forest lands and their influence on soil hydrological features,in order to provide a reference for the rational allocation of forest trees and the evaluation and management of water resources,and provide a theoretical basis for forest soil priority flow process,agricultural irrigation and water saving,soil and groundwater pollution assessment,geological hazard prevention and other related research.The findings can provide scientific explanations for the construction and restoration of regional ecological environment.The study area is located at Wuzuolou forest station near the Miyun Reservoir of Miyun,Beijing,China.Six kinds of typical forest communities were selected as the object of study.Intact soil columns were taken from each forest community.X-ray industrial computed tomography was utilized to scan soil columns,then generating the 3-D images.The analysis of characteristic parameters of macropore and rock fragment was conducted by image processing software.The water infiltration experiment was carried out on all intact soil columns,the soil saturated hydraulic conductivity and soil water content were measured by the fixed water head method and drying method respectively.The characteristic of soil macropores,rock fragments and soil hydrological functions in different forest lands in study area,the coexistence relationship and influencing factors of soil macropores and rock fragments,and the hydrological functions of macropore and rock fragment were found in this study.The main results are summarized as follows:(1)The 3-D structure of image of macropore and rock fragment in undisturbed soil columns can be constructed using industrial CT scanning and image processing technology,many structure parameters can be obtained by further calculation,including number,surface area,volume,diameter,mean tortuosity,mean angle of macropore and number,volume,surface area,diameter of rock fragment.Advanced industrial CT technology can allow an accurate quantification of soil macropore and rock fragment structure.(2)The characteristics of soil macropores in different forest community lands have significant differences.Mixed forest soil have the larger diameter,surface area,network density,and length density of macropores as well as the smallest mean tortuosity of soil macropores than pure forest soil,which means that the development of macropores in mixed is better than that of pure forest.Mixed forest soil not only have the largest number,volume,surface area,but also have the worst degree of macropore bending,which is beneficial to the soil water transport.The difference of macropore characteristics between mixed and pure forests is more significant in the surface soil.The number density,surface area density,volume density and mean diameter all decreased with the depth of soil in all the forest lands,which means that the macropores in the surface soil layer are development better than in the deep soil layer.There is no significant difference of soil rock fragments between the mixed forest land and pure forest land.The content of rock fragments in surface soil layer(0-10cm)is the least,the surface area and volume of rock fragment are all increased with the increment of soil depth.The average particle size of rock fragment is first decreasing and then increasing with the increment of soil depth.(3)There was a significant negative correlation between the number,surface area,volume of macropore and rock fragment in the surface soil(0-10cm)in the study area,but the correlation decreases with the depth of the soil.The volume density of rock fragments has a significant negative correlation with the average inclination angle of the macropores.The volume density,surface area density of rock fragment and mean tortuosity of macropores has a significant positive correlation,it can be seen that the more the volume of rock fragments in the soil,the more bending of the macropores.Soil organic matter content,water content and root length density of mixed forest land were higher than those of pure forest land.The surface soil has higher organic matter content and root length density.There is no significant difference in soil density and sand content between mixed forest and pure forest.The soil organic matter and the root length density have a significant positive correlation with the volume density,surface area density,number density of macropores,and have a significant negative correlation with the mean tortuosity of macropore and the volume density,surface density,number density of rock fragment.Soil root length density and organic matter content are major factors affecting macropores,and rock fragment has indirect effects on macropore characteristics by affecting the accumulation of organic matter and the activity of roots.(4)Soil column infiltration experiments showed that the time of water initial infiltration and reach the saturated hydraulic conductivity between six forest land were no significant difference.The saturated hydraulic conductivity of surface soil in mixed forest land is larger than that in pure forest surface soil,however,with the depth of soil layer,the difference of saturated hydraulic conductivity between mixed forest and pure forest is decreasing.The saturated water conductivity decreases with the depth of the soil in forest land,which is more significant in mixed forest land.There is no significant difference in soil water content between different forestland and different soil layers.The soil saturated hydraulic conductivity was positively correlated with number density,surface area density,volume density of macorpores.The soil saturated hydraulic conductivity was significant negatively correlated with surface area density,volume density of rock fragments and mean tortuosity of macropores.Macropores with pore size>3mm have the greatest influence on soil saturated hydraulic conductivity.Characteristics of macropore and rock fragment are important parameters that influence soil saturated hydraulic conductivity,which codetermined the infiltration capacity of soil water.Among them,the volume density,surface area density of macropore and rock fragment are the most important influencing factors of soil water infiltration.Macropores will increase the effective soil water cross-section and infiltration path to have a positive impact on infiltration,rock fragments will not only reduce the effective cross-sectional and infiltration path directly affect water infiltration,also by increasing tortuosity of macropore,which increases the infiltration path complexity and thus indirectly affect infiltration rate.Compared to the characteristics of rock fragment,the characteristics of macropore have greater influence on soil water infiltration.The soil water content has a significant positive correlation with the surface area density of macropore,which indicates that large surface area of macropore is beneficial to soil water holding.Other characteristics of macropore and rock fragment do not affect soil water content,but the influence of macropore and rock fragment on soil water content is different between different forest lands studied in this work.